45o 
The Progress of the 
[October, 
attractions to their male posterity. Thus, by the secular 
action of this principle, he contends that the gorgeous hues 
and exquisite designs of the wings of butterflies, the train 
of the peacock, the secondary wing-feathers of the Argus 
pheasant, the gorgets and crests of humming-birds, have 
been elaborated. This hypothesis certainly harmonised 
with a considerable number of striking facts previously un- 
explained, and seemed at first glance to agree with many 
more. Mr. Wallace, however, has long held this portion of 
Mr. Darwin’s views to be erroneous, and brings forward 
against it certain exceedingly weighty arguments. In an 
earlier work, with which every naturalist ought to be 
familiar/ he has shown that in female birds the need of 
protection, especially during the season of incubation, has 
repressed those bright colours which would otherwise be 
produced by general laws in both sexes alike. He now 
further argues that high colouration, if not directly due to, 
is yet correlated with, vital intensity. “The very frequent 
superiority of the male bird or insect in brightness or 
intensity of colour, even when the general colouration is 
the same in both sexes, seems to me to be primarily due to 
the greater vigour and activity and the higher vitality of the 
male. The colours of an animal usually fade during 
disease or weakness, while robust health and vigour add to 
their intensity. This is a most important and suggestive 
fact, and one that appears to hold universally. In all 
quadrupeds a dull coat is indicative of ill health or low 
condition, while a glossy coat and sparkling eye are the 
invariable accompaniments of health and energy. The 
same rule applies to the feathers of birds, whose colours 
are only seen in their purity during perfect health ; and a 
similar phenomenon occurs even among insects, for the 
bright hues of caterpillars begin to fade as soon as they be- 
come inactive preparatory to undergoing their transforma- 
tion.” Whenever there is a difference of colour between 
the sexes the male is the darker or more strongly marked, 
and the difference of intensity is most visible during the 
breeding season, when vitality is at its maximum. It is 
undoubtedly true that female birds do exercise a choice, but 
the bulk of the evidence on this point, as collected by Mr. 
Darwin himself, far from proving that such choice is deter- 
mined by colour, points in a directly opposite direction. 
The “ most vigorous, defiant, and mettlesome male ” seems 
to be preferred. These attributes may be, and in a majority 
Contributions to the Theory of Natural Selection. 
